A wireless power transmission or a wireless energy transfer refers to a technology for wirelessly transferring electric energy to desired devices. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electric energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electric toothbrushes or electric razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission employing the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme.
Among them, according to the wireless power transmission employing the resonance, power is supplied from an AC source to generate AC power in a transmission coil and a resonant coil is coupled to the transmission coil to transmit the power through the resonant coil. The wireless power transmission employing the resonance can transmit the power farther than the wireless power transmission employing the electromagnetic induction, but the transmission distance is still short, so the improvement thereof is necessary.